Review Board 3.0 introduced new support for "status updates" which are
used to indicate if a review request is undergoing some automated checks
of some kind (for instance, automated code review or pending builds).
These can have a state (such as "pending", "error", "success", and
others), a summary, optional link, and a review (when there are errors).

This adds a new command for working with status updates, helping to
create/update the state of a status update. This command can be run withrbt status-update.

Testing Done

Debug testing of the --review input on local installation with the following file contents:

It's kind of confusing how this can either be an individual status update or the list of all status updates. I think I'd prefer if the conditional (do we have self.options.sid) is the top-level construct and then we either update or create as appropriate.